The present invention relates to optical devices and more particularly concerns periodic filters and sources.
Wavelength Division Multiplexed (WDM) communication systems offer a high data transmission capacity by allowing multiple laser sources to transmit many high-speed data channels simultaneously over a single fiber, where each channel is transmitted at a unique optical frequency (or wavelength). In order to regularize the frequencies of the channels across telecommunication systems, the industry has adopted a standard which specifies that the nominal optical frequency of every channel should be at an integer multiple or submultiple of 100 GHz. On this uniform frequency grid, typical channel frequencies are therefore 193.100 THz, 193.200 THz, 193.300 THz etc. The frequency of these channels must typically be accurate within 2.5 GHz or 1.25 GHz or even better for correct system operations. During recent years, pressure to put more channels in the same fiber created a need for closer spacing at 50 GHz, 25 GHz, 12.5 GHz and so forth, with an accompanying increase of accuracy.
For a number of reasons, semiconductor lasers currently used in telecommunication systems do not intrinsically generate frequencies that are accurate or stable enough to be used alone in such a frequency grid system, whether they are narrowly or widely tunable lasers. First, current fabrication technologies do not allow to build lasers with a sufficiently accurate relationship between the frequency tuning signal and the actual frequency. Second, the frequency of the laser varies significantly with environmental factors or operating conditions such as injection current or temperature. Third, even if all other parameters are kept constant, the frequency of a laser tends to drift with aging. All these factors can easily detune a laser frequency beyond the accepted limit during its lifetime, and, if used alone, make it unsuitable for operation in a high performance telecommunication system.
Various means have been devised to stabilize the frequency of semiconductor lasers to a predetermined value with a sufficient accuracy. Many of those use an optical frequency reference filter that is sufficiently accurate and stable for telecommunication applications. This reference filter is used to compare the frequency of the laser with the desired predetermined value and generate an error signal which is fed back to the laser to correct its frequency. Once the feedback system is operational and the laser is frequency-locked, the stability of the reference filter is transferred to the laser.
Different optical reference filters have been used in the past to stabilize semiconductor lasers. Some atomic or molecular gases, for instance, exhibit absorption lines in the optical frequency range of telecommunication networks. The frequency of these absorption lines is determined by quantum mechanical laws and are generally extremely precise and stable with respect to environmental factors. These can therefore be considered as absolute reference filters since their accuracy does not depend on a factory calibration. Furthermore, the width of the absorption lines is very narrow, which allows for very sensitive frequency drift detection. Once properly frequency-locked to an absorption line, a laser can display frequency accuracy and stability orders of magnitude better than is required for current telecommunication systems. However, an important drawback of using a gas as a frequency reference is that the absorption lines that serve as references are not evenly spaced, do not occur at exact multiples or submultiples of 100 GHz, and are not present over the whole telecommunication bands.
Various types of optical interferometers or resonators can also be used as optical references to stabilize semiconductor lasers. Devices such as Fabry-Perot etalons or Mach-Zehnder or Michelson interferometers can easily be constructed and integrated into a laser transmitter for the purpose of frequency locking (hence the common name Wavelength Locker). These can be fabricated so that they display a periodic frequency response over a wide range of frequencies depending on the materials used. For instance, the spacing of transmission peaks can be tuned to be near 100 GHz, 50 GHz or whatever spacing is required for telecommunication applications. One drawback of these resonators or interferometric devices is that the accuracy of their frequency response is not absolute, that is, it is not intrinsic to the device but rather depends on their fabrication and installation processes. Further, their frequency response can change with external conditions such as mechanical stresses, temperature and aging. Although very good progresses have been made in constructing and packaging resonators or interferometers that have adequate stability performance for current telecommunication systems, these technologies may not be sufficient for the higher level of accuracy required for very closely spaced frequency grids of the future Dense Wavelength Division Multiplexing (DWDM) systems.
It would be advantageous for telecommunication systems and various kinds of optical instruments to use a device which provides an optical filter displaying a set of evenly spaced transmission peaks over a broad frequency spectrum such as a resonator or an interferometer, but whose frequency response can be known with the accuracy and stability inherent to atomic or molecular gas references. Indeed, DWDM transmitters could use such an absolute periodic reference for internal frequency alignment of the laser on finely spaced ITU sub-channels. Optical monitoring systems could even more be in need of such a calibration-free, low maintenance absolute frequency reference since they must act as a reliable watchdog over a number of channels. Furthermore, optical spectrum measurement instruments and widely tunable laser sources could use this absolute periodic spectrum to calibrate themselves over a wide range of frequencies.
Combining both periodic filters and absolute reference filters into a single apparatus is one step that can be taken to benefit from the properties of both devices. Additional devices and methods can then optionally be added to these optical devices in order to transfer the accuracy of the absolute reference filter to the periodic filter, thereby achieving an absolutely calibrated periodic filter. Such a system could effectively be used as an absolute, calibration-free periodic filter or wavelength locker if the following characteristics are present: a) the periodic filter frequency response is continuously calibrated and stabilized relative to the absolute reference filter; b) the calibration and stabilization procedures are completely automatic and c) the user is able to interrogate the periodic filter without disturbing or being disturbed by the stabilization system.
The general concept of combining an absolute reference filter with a periodic filter to obtain an extended high precision periodic reference is already known in the art. A number of applications of this concept have been previously disclosed in the scientific literature, patent applications and commercial products. These implementations solve some of the problems related to the realization of the absolutely calibrated periodic filter described before, but they still possess some significant drawbacks which are described below, and none presents all the characteristics of a absolute periodic filter that could transparently replace current periodic filters or wavelength lockers.
The C2H2-EX product family from Wavelength Reference, Mulino, Oreg., is one example of a passive (non-tunable) product combining a acetylene gas cell (absolute reference filter) with an etalon (periodic filter) which generates a comb of periodic transmission peaks. FIGS. 1A and 1B (PRIOR ART) show two particular implementations of the general principle behind this product family. In both cases, the etalon (also identified as optical artifact generator) is placed in series with the gas cell. This results in a combined frequency response where gas absorption peaks are superposed to the periodic transmission peaks of the etalon. The outputs are either the resulting optical signals after the filters (FIG. 1A), or the electrical signals of a photodetector which measures the power of the light after the filters (FIG. 1B).
The Wavelength References product has the disadvantage that the etalon frequency response is not tunable and therefore cannot be stabilized actively relative to the gas reference. It does not therefore constitute a periodic frequency reference that can be absolutely calibrated to match the standard telecommunication frequency grids or than can be used as an absolutely calibrated wavelength locker.
When a system or a device comprises both an absolute reference filter and a periodic filter which is frequency-tunable, it is possible to actively control the frequency response of the periodic filter so that it stays in a well known state relative to the absolute reference filter frequency response, thereby achieving absolute calibration of the periodic filter. There are many methods that can be used to stabilize a periodic filter against an absolute reference filter. One such method consists of simultaneously measuring the frequency response of the periodic filter and the absolute filter. Both responses are then compared and the exact frequency response of the periodic filter can be computed. This information is then used to actively tune the periodic filter to maintain its response to a specific value.
Another method to absolutely stabilize a periodic filter consists in using a laser source to simultaneously interrogate the absolute reference filter and the periodic filter. Than can be done by tuning the laser to align its frequency with that of an absorption or transmission feature of the absolute reference filter, and by also tuning the periodic filter in such a way that the laser frequency also coincide with an absorption or transmission feature of the periodic reference. A servo system can then be used to actively maintain the coincidence between the absolute reference and the periodic filter, and thus, ensure that the frequency response of the periodic filter does not move relative to the absolute reference. This second method generally yields a better frequency accuracy.
U.S. Pat. No. 4,856,899 (IWAOKA) describes such a periodic filter that is stabilized relative to an absolute reference filter using a single laser source. Iwaoka describes a tunable light source that is accompanied by a frequency marker system comprising a resonator frequency locked to a stabilized reference laser (FIG. 12 of the above-mentioned patent). Iwaoka also describes that such a reference laser can be obtained by locking a laser on a gas reference. However, this invention does not disclose how the system unambiguously calibrates itself by correctly selecting a specific features of the periodic filter and a specific feature of the absolute reference filter in order to perform the frequency stabilization. Failing to do so prevents the system from implementing a truly absolutely calibrated periodic filter because the frequency response of the stabilized filter cannot be guaranteed as explained below.
Automatically selecting which absorption feature of the periodic filter should be aligned with a specific feature of the absolute reference filter is one fundamental difficulty in implementing an absolute periodic filter. In many situations, many absorption features of the absolute filter are accessible by the laser source. Furthermore, the periodic filter provides a great number of similar transmission features (often called resonance modes, or simply modes) and many of these modes can be aligned with any of those absolute features. Each combination of alignment provides a different periodic filter calibration. In order to obtain a specific, unambiguous periodic filter calibration, there must be a method for selecting exactly which absolute feature and which mode are aligned together. Locking an unknown mode of the periodic filter to a unspecified feature of the absolute reference filter will provide a periodic filter that may be very stable but whose frequency response is still unknown. This would not provide a truly absolutely calibrated periodic filter.
Different solutions have been disclosed in the prior art to address the mode selection problem.
R. Boucher et al.,  less than  less than Calibrated Fabry-Perot Etalon as an Absolute Frequency reference for OFDM Communications) greater than  greater than , IEEE Photonics Technol. Lett., vol. 4, pp. 801-804, July 1992, discloses a method for stabilizing a Fabry-Perot (FP) etalon using absorption lines from Krypton gas around 1300 nm. In this set-up, two laser sources, master and slave, are used to perform the mode selection. The master laser is frequency-locked to a Krypton absorption line and a transmission peak N of the FP is locked to this master reference. Once this is done, a slave laser is locked to a mode N+K, where K is a fixed value. If the correct mode is locked on the master laser, the slave laser light will be close to another absorption line of the Krypton gas and will be absorbed by a specific amount. Using the absorption as a selection criteria, a search can be performed to find which mode N of the FP etalon must be locked to the master laser in order to obtain the desired free spectral range (FSR) from the etalon. Once found, the master laser stays locked on the etalon to keep its frequency response stable, and the slave laser is no longer needed.
U.S. Pat. No. 5,434,877 (CHUNG) also proposes a similar technique. Two laser sources are frequency-locked to specific lines of Krypton around 1550 nm which have a precise frequency difference close to an integer multiple of the desired FSR. Then the FSR of the etalon is tuned until the two reference laser frequencies are precisely matched to two transmission peaks of the etalon, therefore maximizing their output power. This condition, which can be determined by measuring the frequency spectrum of the FP output, indicates that the correct modes have been found. The FP can be locked into its correct position by keeping this maximum power output with a servo loop.
C. Gamache et al., in  less than  less than An Optical Frequency Scale in Exact Multiples of 100 GHz for standardization of Multifrequency Communications greater than  greater than , IEEE Photon. Technol. Lett., vol. 8, pp. 2990-292, Febuary 1996, also describes a similar method to achieve a FSR of exactly 100 GHz. In this set-up lasers #1 and #2 are locked on two selected acetylene line, and lasers #3 and #4 are locked on two selected modes of a Fabry-Perot etalon. The correct tuning of the etalon is achieved when specific beat note frequencies are found between lasers #1 and #3, and between lasers #2 and #4. Once calibrated, the etalon can be frequency-locked with an offset introduced by RF mixing in order to obtain an etalon with a FSR of exactly 100 GHz.
All these methods solve the mode ambiguity problem, but those require the use of two or more lasers in order to correctly select the FP modes during the calibration phase. Once this is done, one or more lasers are no longer needed for normal operation of the stabilized FP. Using these extra lasers for such a limited function is cost-inefficient and increases the size and probability of failure of the system.
The absolute, continuous etalon stabilization method proposed in the prior art can easily be performed manually by a skilled operator, but it is not disclosed how the mode selection can be performed automatically nor how the reference lasers are automatically locked on the correct absorption line of the gas. In the context of a device used in an instrument or in a telecommunications system, all these operations should be automated so that the resulting stabilized etalon (or periodic filter) can be used as an absolute wavelength locker. Furthermore, in order to simplify system design, it would be advantageous that the xe2x80x9cintelligencexe2x80x9d required to implement the automatic frequency calibration of the laser and periodic filter be embedded with those components.
Also known in the art is the PCT application published under no. WO02/31933 (MAY) which discloses several concepts related to the automatic calibration and use of periodic filters. Of particular interest is the embodiment of FIG. 8 in that application, which discloses a tunable laser frequency-locked on an absolutely calibrated etalon. In this embodiment, the output of the laser is split and sent through both an absolute reference filter and an etalon such as a Fabry-Perot filter. The frequency of the laser is scanned and the resulting transmission spectrum of both the absolute reference filter and periodic filter are acquired as a function of the tuning conditions of the laser. A calibration curve for the frequency of the laser as a function of its operating parameters is then obtained, and used to tune the frequency of the laser to a selected value. The operating conditions of the etalon are then also tuned to align one of its transmission peaks with the frequency of the laser. The laser frequency is then locked on the output of the etalon for maintaining this frequency.
A significant drawback of the above system is that the link between the absolute reference filter and the etalon is not continuously maintained. In this system, the absolute reference filter serves only in initially calibrating the laser source. Once the initialization procedure is finished, the output of the laser is locked on the response of the etalon, whose frequency response may itself drift over time since the absolute reference filter no longer plays a role. The above mentioned patent warns that periodic re-calibrations of the system must be performed to ensure a proper alignment of the laser frequency. Since the absolute frequency response of the etalon is not maintained at all times, this therefore does not qualify as a truly absolutely calibrated periodic filter.
In order to use an absolutely calibrated periodic filter as an absolute etalon or wavelength locker, it would be advantageous for the user to be able to pass his light through the filter without perturbing the locking system and without being aware of the stabilization process. The prior art does not address the issue of how this user independence can be effectively obtained.
One possible disadvantage of implementing a stabilized periodic filter is that the required components (absolute reference filter, periodic filter, tuning mechanism, controller etc.) can occupy a significant space. This is a problem especially where the stabilized filter is embedded in a telecommunication transmitter card, or even in a laser module. For those applications, it would be advantageous to have many or all of the required components integrated in a small form-factor device.
A wideband, absolutely stabilized periodic filter could be used with a broadband light source to absolutely calibrate optical spectrum analysis devices or instruments such as Optical Spectrum Analyzers (OSA) or Optical Performance Monitoring (OPM) devices used in telecommunications network surveillance subsystems. To do so, the instrument may measure the transmission spectrum of the absolute periodic filter over a wide frequency range. Since the frequency of each transmission feature of the filter is known absolutely, the instrument can use those features to obtain densely spaced calibration points that would give the instrument a higher accuracy. To simplify the calibration phase, the instrument can use its initial calibration to identify each transmission feature of the periodic filter correctly, or the reference laser signal can be used to establish a distinctive reference point from which all the transmission features of the periodic filter are identified. Widely tunable lasers could also advantageously use an absolute periodic filter to dynamically calibrate their frequency while they are quickly sweeping over large frequency spans.
Finally, it is known in the prior art that simple optical frequency measurement systems can be implemented by measuring the transmission of an optical signal through two or more optical filters and comparing the relative amplitude of each filter to compute the optical signal""s frequency. In such a system, however, the long-term accuracy of the measurement is dictated by the stability of the filters. It would therefore be advantageous for such a system to use an ensemble of filters that are frequency-stabilized to an absolute reference filter in order to obtain enhanced accuracy.
It is therefore an object of the present invention to provide a periodic filter which is stabilized so that it remains absolutely calibrated, which may be used for filtering a user light beam independently of the calibration and stabilization process.
It is another object of the invention to provide a method for filtering a user light beam that provides an absolutely calibrated periodic signal.
It is another object of the present invention to provide an optical source generating an absolutely calibrated broadband periodic signal.
It is yet another object of the present invention to provide a method for absolutely calibrating an optical spectrum analysis device having a broadband frequency response.
It is a preferential object of the present invention to provide a device and method which alleviates the above-explained drawbacks of the prior art.
Accordingly, the present invention concerns an absolutely calibrated optical filtering device for filtering a user light beam, based on a periodic filter having a frequency response including a plurality of substantially regularly-spaced spectral features.
A filter stabilizing assembly is provided for stabilizing the frequency response of the periodic filter at an absolutely calibrated value. The filter stabilizing assembly first includes an absolute reference filter having a transmission spectrum which includes at least one absolutely known absorption feature. A tunable laser source generates a primary light beam at a tunable frequency, at least a portion of this primary light beam being filtered by the absolute reference filter to generate a reference filter beam. At least a portion of the primary light beam is filtered by the periodic filter to generate a periodic filter beam. The filter stabilizing assembly also includes means for separately obtaining a reference filter signal and a periodic filter signal from the reference filter and periodic filter beams, respectively. Laser locking means are connected to the tunable laser source and use the reference filter signal for locking the frequency of the tunable laser source relative to a selected one of the absorption features of the absolute reference filter. Finally, filter stabilizing means are connected to the periodic filter and use the periodic filter signal for stabilizing the frequency response of the periodic filter by locking one of the spectral features thereof relative to the frequency of the tunable laser source.
The filtering device also includes a user input receiving the user light beam and propagating the same through the periodic filter, thereby generating a filtered user light beam according to the frequency response of the periodic filter. A user output is also provided, outputting the filtered user light beam independently of the periodic filter beam.
In accordance with another aspect of the present invention, there is also provided a method for filtering of a user light beam. This method includes the following steps:
A- providing a periodic filter having a frequency response including a plurality of substantially regularly-spaced spectral features;
B- stabilizing the frequency response of this periodic filter at an absolutely calibrated value, the stabilizing comprising the steps of:
a) generating a primary light beam at a tunable frequency with a tunable laser source;
b) filtering at least a portion of the primary light beam through an absolute reference filter having a transmission spectrum which includes at least one absolutely known absorption feature to generate a reference filter beam;
c) filtering at least a portion of the primary light beam through the periodic filter to generate a periodic filter beam;
d) separately obtaining a reference filter signal and a periodic filter signal from the reference filter and periodic filter beams, respectively;
e) locking the frequency of the tunable laser source relative to a selected one of the absorption features of the absolute reference filter using the reference filter signal; and
f) stabilizing the frequency response of the periodic filter using the periodic filter signal by locking a selected spectral feature of the frequency response relative to the frequency of the tunable laser source;
C- receiving the user light beam and propagating the same through the periodic filter, thereby generating a filtered user light beam according to the frequency response of the periodic filter; and
D- outputting the filtered user light beam independently of the periodic filter beam.
In accordance with yet another aspect of the present invention, there is also provided an absolutely calibrated optical source for generating a broadband periodic light beam.
The source first includes a periodic filter having a frequency response including a plurality of substantially regularly-spaced spectral features. A filter stabilizing assembly is provided. It includes an absolute reference filter having a transmission spectrum which includes at least one absolutely known absorption feature. A tunable laser source is provided and generates a primary light beam at a tunable frequency, at least a portion of this primary light beam being filtered by the absolute reference filter to generate a reference filter beam, and at least a portion of the primary light beam being filtered by the periodic filter to generate a periodic filter beam. Means are provided for separately obtaining a reference filter signal and a periodic filter signal from the reference filter and periodic filter beams, respectively. Laser locking means are connected to the tunable laser source and use the reference filter signal for locking the frequency of the tunable laser source relative to a selected one of the absorption features of the absolute reference filter, thereby generating a stabilized laser signal. Filter stabilizing means are connected to the periodic filter and use the periodic filter signal for stabilizing the frequency response of the periodic filter by locking one of the spectral features thereof relative to the frequency of the stabilized laser signal.
The optical source also includes a broadband light source generating a broadband light beam. Means are provided for propagating the broadband light beam through the periodic filter, thereby generating the broadband periodic light beam according to the frequency response of the periodic filter. An output outputs the broadband periodic light beam.
In preferred embodiments, the absolutely calibrated optical source above may output the periodic broadband signal combined with the periodic filter beam, or independently.
Advantageously, the absolutely calibrated optical source above may be used for absolutely calibrating an optical spectrum analysis device having a broadband frequency response. The corresponding method includes:
a) providing an absolutely calibrated optical source as above; and
b) using the broadband periodic light beam to calibrate a plurality of points of the frequency response.
According to an alternative embodiment, this method may also involve the following steps:
a) providing an absolutely calibrated optical source as above outputting the periodic broadband beam and periodic filter beam combined;
b) using the periodic filter beam to absolutely calibrate a first point of the frequency response of said spectrum analyzer; and
c) using the broadband periodic light beam relative to said first point of the frequency response of said spectrum analysis device to calibrate a plurality of points of said frequency response.
Other features and advantages of the present invention will be better understood upon reading of preferred embodiments thereof with reference to the appended drawings.